Download Metaphase I

• Overview: Hereditary Similarity and Variation
• Living organisms
– Are distinguished by their ability to reproduce
their own kind
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• Heredity
– Is the transmission of traits from one generation to
the next
• Variation
– Shows that offspring differ somewhat in
appearance from parents and siblings
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• Genetics
– Is the scientific study of heredity and hereditary
variation
– Concept 13.1: Offspring acquire genes from
parents by inheriting chromosomes
• Genes
– Are the units of heredity
– Are segments of DNA
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• Each gene in an organism’s DNA
– Has a specific locus on a certain chromosome
• We inherit
– One set of chromosomes from our mother and
one set from our father
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Comparison of Asexual and Sexual Reproduction
• In asexual reproduction
– One parent produces genetically identical
offspring by mitosis
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• In sexual reproduction
– Two parents give rise to offspring that have
unique combinations of genes inherited from
the two parents
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• Concept 13.2: Fertilization and meiosis
alternate in sexual life cycles
• A life cycle
– Is the generation-to-generation sequence of
stages in the reproductive history of an
organism
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Sets of Chromosomes in Human Cells
• In humans
– Each somatic cell has 46 chromosomes, made
up of two sets
– One set of chromosomes comes from each
parent
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• A karyotype
– Is an ordered, visual representation of the
chromosomes in a cell
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• Homologous chromosomes
– Are the two chromosomes composing a pair
– Have the same characteristics
– May also be called autosomes
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• Sex chromosomes
– Are distinct from each other in their
characteristics
– Are represented as X and Y
– Determine the sex of the individual, XX being
female, XY being male
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• A diploid cell
– Has two sets of each of its chromosomes
– In a human has 46 chromosomes (2n = 46)
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• In a cell in which DNA synthesis has occurred
– All the chromosomes are duplicated and thus
each consists of two identical sister chromatids
Key
Maternal set of
chromosomes (n = 3)
2n = 6
Paternal set of
chromosomes (n = 3)
Two sister chromatids
of one replicated
chromosome
Centromere
Figure 13.4
Two nonsister
chromatids in
a homologous pair
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Pair of homologous
chromosomes
(one from each set)
Figure 13.3
APPLICATION
TECHNIQUE
Pair of homologous
duplicated chromosomes
Centromere
Sister
chromatids
Metaphase
chromosome
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5 m
Figure 13.3a
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Figure 13.3b
Pair of homologous
duplicated chromosomes
Centromere
Sister
chromatids
Metaphase
chromosome
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5 m
Figure 13.3c
5 m
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• Unlike somatic cells
– Gametes, sperm and egg cells are haploid
cells, containing only one set of chromosomes
• At sexual maturity
– The ovaries and testes produce haploid
gametes by meiosis
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• During fertilization
– These gametes, sperm and ovum, fuse,
forming a diploid zygote
• The zygote
– Develops into an adult organism
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• The human life cycle
Key
Haploid gametes (n = 23)
Haploid (n)
Diploid (2n)
Ovum (n)
Sperm
Cell (n)
FERTILIZATION
MEIOSIS
Ovary
Testis
Mitosis and
development
Figure 13.5
Multicellular diploid
adults (2n = 46)
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Diploid
zygote
(2n = 46)
The Variety of Sexual Life Cycles
• The three main types of sexual life cycles
– Differ in the timing of meiosis and fertilization
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• In animals
– Meiosis occurs during gamete formation
– Gametes are the only haploid cells
Key
Haploid
Diploid
n
n
Gametes
n
MEIOSIS
FERTILIZATION
Zygote
2n
Figure 13.6 A
Diploid
multicellular
organism
2n
Mitosis
(a) Animals
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• Plants and some algae
– Exhibit an alternation of generations
– The life cycle includes both diploid and haploid
multicellular stages Haploid multicellular
organism (gametophyte)
n
Mitosis
n
Mitosis
n
n
n
Spores
Gametes
MEIOSIS
Diploid
multicellular
organism
(sporophyte)
Figure 13.6 B
2n
(b) Plants and some algae
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FERTILIZATION
2n
Mitosis
Zygote
• In most fungi and some protists
– Meiosis produces haploid cells that give rise to
a haploid multicellular adult organism
– The haploid adult carries out mitosis,
producing cells that will become gametes
Haploid multicellular
organism
Mitosis
n
Mitosis
n
n
n
Gametes
MEIOSIS
FERTILIZATION
2n
Figure 13.6 C
Zygote
(c) Most fungi and some protists
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n
• Concept 13.3: Meiosis reduces the number of
chromosome sets from diploid to haploid
• Meiosis
– Takes place in two sets of divisions, meiosis I
and meiosis II
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• Meiosis I
– Reduces the number of chromosomes from
diploid to haploid
• Meiosis II
– Produces four haploid daughter cells
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• Division in meiosis I occurs in four phases
– Prophase I
– Metaphase I
– Anaphase I
– Telophase I and cytokinesis
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2011 Pearson
Education, Inc.
Figure 13.8
MEIOSIS I: Separates sister chromatids
MEIOSIS I: Separates homologous chromosomes
Prophase I
Metaphase I
Centrosome
(with centriole pair)
Sister
chromatids
Chiasmata
Telophase I and
Cytokinesis
Anaphase I
Duplicated homologous
chromosomes (red and blue)
pair and exchange segments;
2n  6 in this example.
Anaphase II
Telophase II and
Cytokinesis
Centromere
(with kinetochore)
Metaphase
plate
Cleavage
furrow
Fragments
of nuclear
envelope
Metaphase II
Sister chromatids
remain attached
Spindle
Homologous
chromosomes
Prophase II
Homologous
chromosomes
separate
Microtubule
attached to
kinetochore
Chromosomes line up
by homologous pairs.
Each pair of homologous
chromosomes separates.
During another round of cell division, the sister chromatids finally separate;
four haploid daughter cells result, containing unduplicated chromosomes.
Sister chromatids
separate
Two haploid cells
form; each chromosome
still consists of two
sister chromatids.
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Haploid daughter
cells forming
Figure 13.8a
Prophase I
Centrosome
(with centriole pair)
Sister
chromatids
Chiasmata
Spindle
Telophase I and
Cytokinesis
Anaphase I
Metaphase I
Sister chromatids
remain attached
Centromere
(with kinetochore)
Metaphase
plate
Fragments
Homologous
chromosomes of nuclear
envelope
Homologous
chromosomes
separate
Microtubule
attached to
kinetochore
Each pair of homologous
chromosomes separates.
Chromosomes line up
Duplicated homologous
chromosomes (red and blue) by homologous pairs.
pair and exchange segments;
2n  6 in this example.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Cleavage
furrow
Two haploid
cells form; each
chromosome
still consists
of two sister
chromatids.
Prophase I
• Prophase I typically occupies more than
90% of the time required for meiosis
• Chromosomes begin to condense
• In synapsis, homologous chromosomes
loosely pair up, aligned gene by gene
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2011 Pearson
Education, Inc.
• In crossing over, nonsister chromatids
exchange DNA segments
• Each pair of chromosomes forms a tetrad, a
group of four chromatids
• Each tetrad usually has one or more
chiasmata, X-shaped regions where
crossing over occurred
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2011 Pearson
Education, Inc.
Metaphase I
• In metaphase I, tetrads line up at the metaphase
plate, with one chromosome facing each pole
• Microtubules from one pole are attached to the
kinetochore of one chromosome of each tetrad
• Microtubules from the other pole are attached to
the kinetochore of the other chromosome
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2011 Pearson
Education, Inc.
Anaphase I
• In anaphase I, pairs of homologous
chromosomes separate
• One chromosome moves toward each pole,
guided by the spindle apparatus
• Sister chromatids remain attached at the
centromere and move as one unit toward the
pole
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2011 Pearson
Education, Inc.
Telophase I and Cytokinesis
• In the beginning of telophase I, each half of
the cell has a haploid set of chromosomes;
each chromosome still consists of two sister
chromatids
• Cytokinesis usually occurs simultaneously,
forming two haploid daughter cells
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2011 Pearson
Education, Inc.
• In animal cells, a cleavage furrow forms; in
plant cells, a cell plate forms
• No chromosome replication occurs
between the end of meiosis I and the
beginning of meiosis II because the
chromosomes are already replicated
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©Copyright
2011 Pearson
Education, Inc.
• Division in meiosis II also occurs in four phases
– Prophase II
– Metaphase II
– Anaphase II
– Telophase II and cytokinesis
• Meiosis II is very similar to mitosis
© 2005 Pearson Education, Inc. publishing as Benjamin Cummings
©Copyright
2011 Pearson
Education, Inc.
Figure 13.8b
Prophase II
Metaphase II
Anaphase II
Telophase II and
Cytokinesis
During another round of cell division, the sister chromatids finally separate;
four haploid daughter cells result, containing unduplicated chromosomes.
Sister chromatids
separate
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Haploid daughter
cells forming
Prophase II
• In prophase II, a spindle apparatus forms
• In late prophase II, chromosomes (each
still composed of two chromatids) move
toward the metaphase plate
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©Copyright
2011 Pearson
Education, Inc.
Metaphase II
• In metaphase II, the sister chromatids are
arranged at the metaphase plate
• Because of crossing over in meiosis I, the
two sister chromatids of each chromosome
are no longer genetically identical
• The kinetochores of sister chromatids
attach to microtubules extending from
opposite poles
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©Copyright
2011 Pearson
Education, Inc.
Anaphase II
• In anaphase II, the sister chromatids
separate
• The sister chromatids of each chromosome
now move as two newly individual
chromosomes toward opposite poles
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©Copyright
2011 Pearson
Education, Inc.
A Comparison of Mitosis and Meiosis
• Meiosis and mitosis can be distinguished from
mitosis
– By three events in Meiosis l
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• A comparison of mitosis and meiosis
MITOSIS
MEIOSIS
Chiasma (site of
crossing over)
Parent cell
(before chromosome replication)
MEIOSIS I
Prophase I
Prophase
Chromosome
replication
Duplicated chromosome
(two sister chromatids)
Chromosome
replication
Tetrad formed by
synapsis of homologous
chromosomes
2n = 6
Metaphase
Chromosomes
positioned at the
metaphase plate
Anaphase
Telophase
Sister chromatids
separate during
anaphase
2n
Tetrads
positioned at the
metaphase plate
Homologues
separate
during
anaphase I;
sister
chromatids
remain together
Metaphase I
Anaphase I
Telophase I
Haploid
n=3
Daughter
cells of
meiosis I
2n
MEIOSIS II
Daughter cells
of mitosis
n
n
n
Daughter cells of meiosis II
Figure 13.9
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Sister chromatids separate during anaphase II
n
• Concept 13.4: Genetic variation produced in
sexual life cycles contributes to evolution
• Reshuffling of genetic material in meiosis
– Produces genetic variation
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Origins of Genetic Variation Among Offspring
• In species that produce sexually
– The behavior of chromosomes during meiosis
and fertilization is responsible for most of the
variation that arises each generation
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Independent Assortment of Chromosomes
• Homologous pairs of chromosomes
– Orient randomly at metaphase I of meiosis
• In independent assortment
–
Each pair of chromosomes sorts its maternal and paternal
homologues into daughter cells independently of the other pairs
• Crossing over
–
Produces recombinant chromosomes that carry genes derived
from two different parents
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Random Fertilization
• The fusion of gametes
– Will produce a zygote with any of about 64
trillion diploid combinations
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Evolutionary Significance of Genetic Variation
Within Populations
• Genetic variation
– Is the raw material for evolution by natural
selection
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• Mutations
– Are the original source of genetic variation
• Sexual reproduction
– Produces new combinations of variant genes,
adding more genetic diversity
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